1. Field of the Invention
The present invention relates to an inverter system, and more particularly to the inverter system with a variable input gain apparatus and a method for the same.
2. Description of the Prior Art
An inverter is used to convert the direct current (DC) power to the alternating current (AC) power. Therefore, the inverters have been widely applied in solar electricity generating system, motor scooter, uninterruptible power supply, and so on. In an inverter, a voltage/frequency (V/f) control is used to provide variable speed control for motors and consequently inverters can be advantageously applied in refrigeration and air conditioning system, high speed rail system, mass rapid transit system, and so on to promote use efficiency of motors so as to increase energy efficiency.
Reference is made in FIG. 1. It is a block diagram of an inverter system of the prior art. A host 10A delivers an analog control signal to an inverter 30A through an analog input/output module 20A. The inverter 30A includes an analog to digital converter (not shown) to convert the analog control signal into a digital signal, which is processed by a microcontroller unit 300A so as to control the inverter 30A.
At present, the inverters provide control on the frequency of output voltage according to an analog input voltage (between 0 to 10 volts).
However, the analog input voltage has to be processed through a proportional amplifier and then entered into analog input pins of the microcontroller unit 300A. Generally speaking, an operating voltage of the microcontroller unit 300A is +5 volts or +3.3 volts, so the proportional gain is usually a fixed scale-down gain value. Reference is made in FIG. 2. It is a prior art circuit diagram for processing an analog input signal of an analog input/output module. The analog input voltage signal VAI (between 0 to 10 volts) is proportionally transformed into a voltage between 0 to 5 volts through a non-inverting amplifier 22A with a fixed proportional gain P (P=0.5) and the transformed voltage is sent into analog input pins of the microcontroller unit 300A and converted by the analog-to-digital converter unit 24A.
However, in the above-mentioned circuit, accuracy rate of the microcontroller unit 300A is not high when the analog input voltage is lower than +1 volts. FIG. 3 shows a conversion result performed by the circuit in FIG. 2, wherein an analog input voltage signal is converted into a digital count value with 10-bit resolution. A voltage variation is 5 millivolts (mV) (see formula 1) for a least significant bit (LSB) of the analog-to-digital converter unit 24A when a reference voltage VREF thereof is 5.12 volts. If an error of the analog-to-digital converter unit 24A is ±3 LSB, a digital count value N acquired by the microcontroller unit 300A is 100±3 (see formula 2) when the analog input voltage signal is 1.00 volt.
A width of the LSB is 5 mV:
                                          5.12            ⁢                          (              volt              )                                1024                =                  5          ⁢                      (            mV            )                                              (                  formula          ⁢                                          ⁢          1                )            
The digital count value N is calculated:
                    N        =                                            V              AI                        ×            P            ×                          1024                              V                REF                                              =                                    1              ×                              1                2                            ×                              1024                5.12                                      =            100                                              (                  formula          ⁢                                          ⁢          2                )            
Hence, the analog input voltage signal VAI can be inversely calculated from the digital count value N:
                              V          AI                =                                            N              ×                              V                REF                            ⁢                              /                            ⁢              1024                        P                    ⁢                      +            _                    ⁢                                                    (                                  3                  +                                      1                    ⁢                                          /                                        ⁢                    2                                                  )                            ×                              V                REF                            ⁢                              /                            ⁢              1024                        P                                              (                  formula          ⁢                                          ⁢          3                )            
The analog input voltage signal is 1±0.035 volts when the digital count value N acquired by the microcontroller unit 300A is 100. The error of the microcontroller unit 300A is amplified due to the fixed proportional gain P (P=0.5<1), therefore, a maximum error is 0.035 volts. The error rate increases when the analog input voltage is lower; in another word, the accuracy rate decreases when the analog input voltage is lower so that microcontroller unit 300A can't be accurately controlled.
Table 1 obviously shows that the accuracy rate of the microcontroller unit 300A gradually decreases when the analog input voltage decreases.
TABLE 1Error rate and accuracy rate of the microcontroller for estimating ananalog input voltage signalAnalog input voltage signalError rate of theAccuracy rate of the(VAI)microcontrollermicrocontroller1.003.5%96.5%0.804.4%95.6%0.605.8%94.2%0.408.8%91.3%0.2017.5%82.5%0.1035.0%65.0%
Hence, in order to increase accuracy rate of the microcontroller unit 300A when the analog input voltage signal is low, a switch circuit with a variable input gain is desirable. The switch circuit with variable input gain selectively increases the analog input voltage to the analog-to-digital converter so as to decrease error rate of the microcontroller unit 300A.